The invention concerns a wheel anti-skid brake control system for fluid pressure brakes, particularly for motor vehicles, wherein a sensor device that scans and evaluates the dynamic wheel behavior during braking is assigned to at least one wheel of an axle, and its output signals influence the brake pressure at both wheels of a common axle via an electronic evaluating circuit.
Known wheel anit-skid brake systems are characterized by the fact that the front wheels of a vehicle have modulator devices which alter the brake pressure in the wheel cylinders during braking, depending on the rotational behavior of the front wheels. The brake lines from these modulator devices to the wheel brake cylinders of the front wheels are connected with the brake cylinders of the back wheels via auxiliary brake lines. In this way, the braking force is concurrently modulated on all braking wheels of the vehicle substantially the same.
In vehicles with a very short wheel base, such a brake pressure control system can adequately control the pressure. However, this system cannot be used in vehicles with a long wheel base, particularly multi-axle vehicles, because in the case of compressed air brake systems such as those used in commerical vehicles, for example, long brake lines do not permit rapid adjustment of the brake pressure, nor can the axle load fluctuations resulting from varying load conditions be accounted for. Another disadvantage of this system is that, due to the arrangement of the devices that control the brake pressure according to the rotational behavior only at the front wheels of the vehicles, the rotational behavior of the rear wheels, which can briefly encounter a different coefficient of friction than the front wheels, is not taken into consideration. For example, when the friction is high at the front wheels and low at the back wheels, the back wheels are overbraked and momentarily locked, which makes the vehicle unstable. On the other hand, when the friction is low at the front wheels and high at the back wheels, only a low brake pressure is sent into the brake cylinders of the front wheels and hence, into those of the rear wheels, so that the total braking force is small, and the braking distance is needlessly increased.
It is also conceivable, for example, that sensor devices are arranged on diagonally opposed wheels of different axles of a vehicle and their output signals influence the brake pressure at all braking wheels of the vehicle via a single evaluating circuit and a modulator valve unit, with the output signals of both sensor devices being determined by an evaluating circuit for the purpose of brake control, such that only one output signal is used at any given time for control. However, such a wheel anti-skid brake control system has the disadvantage that the output signal of the signal device of the wheel that first indicates a tendency to lock determines the brake pressure of all the wheels. For example, when the right front wheel of a heavily loaded vehicle encounters less friction on the road than the left rear wheel, the brake pressure is reduced at all wheels that are braked. Thus, maximum utilization of the available braking force at the wheels encountering the higher friction is prevented, the result being a longer braking distance. A load modified brake force or anti-skid brake control that accounts for the varying axle loads is impossible with such a system unless expensive auxiliary equipment is used.